One of the electrical connections made to a laser diode chip is typically a wire bond connection, while the other electrical connection is typically provided by way of the substrate to which that chip is bonded. That bond with the substrate is typically a solder bond, and for this purpose a hard solder, typically a substantially eutectic gold-tin solder, is generally employed because it provides a bond significantly less susceptible to creep than an equivalent bond effected using soft solder. The substrate to which the chip is bonded also performs the function of a heat-sink, or of a low thermal impedance pathway to a heat-sink. A laser diode chip typically comprises a number of epitaxial layers grown upon a single crystal slice in which the aggregate thickness of the epitaxial layers is much less than the thickness of the single crystal slice, even after that slice has been thinned. Since the heat that is generated by operation of the laser is heat generated within the epitaxial layers, it is generally preferred to bond laser diode chips epitaxial layers face-down on to their heat-sink substrates. A feature of laser diode chip manufacture is that the thickness of the epitaxial layers are typically held within closer tolerances than that of the thickness of the thinned slices upon which those layers are grown. Accordingly, bonding laser chips epitaxial layers face down is liable to facilitate the achieving of a close tolerance in the height of the laser emission above the surface of the substrate than when the chips are bonded expitaxial layers face-up. This can be of particular benefit when the surface of the substrate is also used as a reference surface with which one or more other optical elements are aligned for optical coupling with the laser emission. For instance it facilitates efficient coupling of the laser emission with an optical fibre that is aligned by being secured in a V-groove crystallographically etched in a single crystal silicon substrate, such as that described in U.S. Pat. No. 5,522,000.
One of the structures of laser diode chip that it is desired to bond to substrates is the ridge-structure laser. In such a laser a pair of parallel troughs are formed in the epitaxially deposited layers, defining between these troughs an intervening ridge is which lasing occurs. The ridge performs a lateral optical waveguide function and, by restricting the area over which electrical contact is made with the topmost epitaxial layer, exercises a degree of lateral confinement upon the injection of minority carriers into the active layer of the laser.
In a ridge structure laser, as in other types of laser diode, minority carriers injected into the active layer provide optical gain within the semiconductor material, and some form of optical positive feedback mechanism is then required to produce laser action. Such positive feedback can be provided by Fresnel reflection at the end facets of the laser chip, resulting in what is termed Fabry Perot feedback, such lasers often being referred to as FP lasers. Reflection can alternatively be provided by a Bragg grating structure. Thus end-facet Fresnel reflection at either or both ends of the chip can be suppressed, and its place taken by a Bragg grating structure near that end, thereby producing a type of laser structure known as a Distributed Bragg Reflection laser or DBR laser. Alternatively a Bragg grating structure can be formed that extends most of the length from one Fresnel reflection suppressed end facet to the other thereby producing a type of laser structure known as a Distributed Feed-Back laser or DFB laser.
When solder bonding a laser diode chip to a substrate, it might be thought best to have the solder bond extend across the full extent of the chip face because this should minimise the thermal impedance presented by the solder. In practice it has been found preferable, at least in certain circumstances, to bond ridge-structure FP laser diode chips expitaxial layers face-down on substrates using parallel stripes of solder, each extending substantially the full length of a chip, but having a width that is small compared with the full width of the chip. In respect of an individual laser diode chip, one of these stripes is arranged to register with the ridge, this stripe being flanked by at least two others that are provided to register with the face of the chip on either side of the ridge-and-troughs structure. Such an arrangement is for instance employed in the 1500 nm Mini-DIL TRANSMITTER marketed by Nortel Limited. The use of these stripes is found to lead to a more consistent height in the bonded laser of the expitaxial layers above the substrate. In these marketed devices only the central solder stripe that registers with the chip ridge makes electrical connection with an electrical interconnection connection layer of the substrate. The other solder stripes are electrically isolated from that interconnection layer, and so perform no electrical function; their function is that of providing mechanical stability and additional thermal conduction paths between the chip and the substrate.
Adopting the same procedure in respect of mounting ridge-structure DFB laser diode chips on substrates has been found to give rise to unacceptable degradation of laser performance. One solution to this problem is to bond such chips the other way up, i.e. solder them to their substrates epitaxial layers face-up. However, for many applications this is far from an ideal solution because of the increased thermal impedance thereby presented between the active region in the chip and the substrate to which that chip is bonded, and additionally because the height of the active layer above the substrate surface is liable to vary more from chip to chip because of differences in thickness, after thinning, of the semiconductor slices upon which the expitaxial layers of those chips have been deposited.